System and method for stabilizing mode locked swept laser for OCT medical imaging

ABSTRACT

An optical coherence analysis system uses a laser swept source that is constrained to operate in a stable mode locked condition by modulating a drive current to the semiconductor optical amplifier as function of wavelength or synchronously with the drive voltage of the laser&#39;s tunable element based on stability map for the laser.

BACKGROUND OF THE INVENTION

Optical coherence analysis relies on the use of the interferencephenomena between a reference wave and an experimental wave or betweentwo parts of an experimental wave to measure distances and thicknesses,and calculate indices of refraction of a sample. Optical CoherenceTomography (OCT) is one example technology that is used to performhigh-resolution cross sectional imaging. It is often applied to imagingbiological tissue structures, for example, on microscopic scales in realtime. Optical waves are reflected from an object or sample and acomputer produces images of cross sections of the object by usinginformation on how the waves are changed upon reflection.

Fourier domain OCT (FD-OCT) currently offers the best performance formany applications. Moreover, of the Fourier domain approaches,swept-source OCT has distinct advantages over techniques such asspectrum-encoded OCT because it has the capability of balanced andpolarization diversity detection. It has advantages as well for imagingin wavelength regions where inexpensive and fast detector arrays, whichare typically required for spectrum-encoded FD-OCT, are not available.

In swept source OCT, the spectral components are not encoded by spatialseparation, but they are encoded in time. The spectrum is eitherfiltered or generated in successive frequency steps and reconstructedbefore Fourier-transformation. Using the frequency scanning sweptsource, the optical configuration becomes less complex but the criticalperformance characteristics now reside in the source and especially itsfrequency tuning speed and accuracy.

High speed frequency tuning for OCT swept sources is especially relevantto in vivo imaging where fast imaging reduces motion-induced artifactsand reduces the length of the patient procedure. It can also be used toimprove resolution.

The swept sources for OCT systems have typically been tunable lasers.The advantages of tunable lasers include high spectral brightness andrelatively simple optical designs. A tunable laser is constructed from again medium, such as a semiconductor optical amplifier (SOA) that islocated within a resonant cavity, and a tunable element such as arotating grating, grating with a rotating mirror, or a Fabry-Perottunable filter. Currently, some of the highest tuning speed lasers arebased on the laser designs described in U.S. Pat. No. 7,415,049 B1,entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element,by D. Flanders, M. Kuznetsov and W. Atia. The use ofmicro-electromechanical system (MEMS) Fabry-Perot tunable filterscombines the capability for wide spectral scan bands with the low mass,high mechanical resonant frequency deflectable MEMS membranes that havethe capacity for high speed tuning.

Certain tradeoffs in laser design, however, can be problematic for OCTsystems. Generally, shorter laser cavities translate to higher potentialtuning speeds, since laser oscillation must build up anew fromspontaneous emission when the laser is tuned. Thus, round-trip traveltime for the light in the laser cavities should be kept low so that thisbuild up occurs quickly. Short laser cavities, however, create problemsin terms of the spectral spacing of the longitudinal cavity modes of thelaser. That is, lasers can only produce light at frequencies which areinteger multiples of the cavity mode spacing since the light mustoscillate within the cavities. Shorter cavities result in fewer and morewidely spaced modes. This results in greater mode hopping noise as thelaser is tuned over these discrete cavity modes. So, when designing anOCT laser, there is typically a need to choose between low noise andhigh speed.

Research with swept tunable lasers has shown that when they are operatedat high sweep rates they tend to operate in a mode locked regime. In amode locked regime optical power of the laser varies on a time scale ofthe cavity roundtrip time as one or more optical pulses travel in thelaser cavity, as is found in a traditional mode locked laser. The pulserepetition rate is close to the laser cavity roundtrip time or to atypically small, say a factor of 2 to 10, multiple. Since this modelocking arises from frequency tuning of the laser, it is termed sweptmode locking.

This swept mode locked regime can have the effect of actuallyfacilitating the high-speed tuning of the laser. A four-wave mixingeffect red shifts the wave in the laser cavity. This facilitates thetuning to lower optical frequencies. See A. Bilenca, S. H. Yun, G. J.Tearney, and B. E. Bouma, “Numerical study of wavelength-sweptsemiconductor ring lasers: the role of refractive-index nonlinearitiesin semiconductor optical amplifiers and implications for biomedicalimaging applications”, OPTICS LETTERS/Vol. 31, No. 6, Mar. 15, 2006.

Problems, however, often arise when tuning to higher optical frequenciesand also during very high speed tuning. Generally, this tuning tends tobe more unstable. Some of these instabilities probably result from thefact that the laser cavity is changing through the process of tuning,and thus the characteristics that instigate the swept mode locking alsochange. As a result, the lasers can flip between different swept modelocked regimes during a single frequency scan of the tunable laser. Forexample, during the sweep, the number of pulses circulating in thecavity can change, causing the lasers to behave chaotically andunpredictably as they move between the different regimes. The differentregimes can further result in different performance characteristics asthe tunable lasers relate to the OCT systems in which they operate.

SUMMARY OF THE INVENTION

The present invention concerns a swept tunable laser source. During itsswept operation, it is constrained to operate in a controlled modelocked regime preferably by controlling the drive current to the laser'sgain element, typically a semiconductor optical amplifier.

This has the effect of stabilizing the emission characteristics of thelaser and avoids noisy disruptions due to uncertainty or flips in thenumber of pulses circulating in the cavity. Instead, the mode lockingsystem stabilizes the pulsation behavior of the laser by modulating again, for example, of the cavity of the laser synchronously with thesweeping of the laser's tunable element, e.g., Fabry-Perot tunablefilter or grating.

In other embodiments described below, the stabilization is accomplishedby modulating a lossy element within the cavity.

In general according to one aspect, the invention features an opticalcoherence imaging method, comprising providing a laser swept source togenerate a swept optical signal, modulating a drive signal for a gainelement as the swept optical signal is swept through a scan band,transmitting the swept optical signal to an interferometer having areference arm and a sample arm, in which a sample is located, combiningthe swept optical signal returning from the sample arm and the referencearm to generate an interference signal, detecting the interferencesignal, and generating image information of the sample from the detectedinterference signal.

In embodiments, modulating the drive signal comprises modulating a biascurrent to a semiconductor optical amplifier. This is preferably donesynchronously with a tunable element drive signal.

In general according to another aspect, the invention features anoptical coherence analysis system, comprising a swept laser source forgenerating a swept optical signal that is frequency tuned over a tuningband, a controller for modulating a drive signal for a gain element asthe swept optical signal is swept through a scan band, an interferometerfor dividing the swept optical signal between a reference arm and asample arm leading to a sample, and a detector system for detecting aninterference signal generated from the swept optical signal from thereference arm and from the sample arm.

In general according to another aspect, the invention features anoptical swept source control method, comprising providing a laser sweptsource to generate a swept optical signal and modulating a drive signalfor a gain element as the swept optical signal is swept through a scanband as a function of wavelength to control a mode locked operation.

In general, according to another aspect, the invention features a sweptlaser system comprising a swept laser source for generating a sweptoptical signal that is frequency tuned over a tuning band and acontroller for modulating a drive signal for a gain element as the sweptoptical signal is swept through a scan band to control the mode lockedoperation.

In general according to another aspect, the invention features anoptical swept source control method, comprising providing a laser sweptsource to generate a swept optical signal, generating a stability map asa function of wavelength for the laser swept source, and modulating adrive signal for a gain element based on the stability map.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the same orsimilar parts throughout the different views. The drawings are notnecessarily to scale; emphasis has instead been placed upon illustratingthe principles of the invention. Of the drawings:

FIG. 1 contains five plots of experimental data on a common timescale inmicroseconds: clock frequency in MegaHertz, clock jitter in percent,laser power output in arbitrary units, relative intensity noise (RIN)(dBc/Hz), and a spectrogram showing the frequency content vs. time ofthe laser's instantaneous power output, illustrating a tunable lasersource exhibiting swept mode locking during scanning over the tuningband but without stabilization;

FIG. 2A contains four plots from a computer simulation on a commontimescale in picoseconds: optical power for light exiting the SOA 410and the Fabry-Perot tunable filter 412, the instantaneous opticalfrequency change of the pulses in GigaHertz (GHz), the gain from the SOA410 and cavity loss, and the bias current to the SOA 410, and FIG. 2B isa plot of normalized fringe amplitude from a test interferometer as afunction of depth in millimeters, illustrating a tunable laser sourceexhibiting swept mode locking during scanning over the tuning band butwithout stabilization;

FIG. 3 is a schematic view of an OCT system incorporating the stabilizedmode locked swept laser according to an embodiment of the invention;

FIG. 4 is a schematic diagram of a passively mode-locked laser sweptsource for optical coherence analysis having controller for modulating adrive signal for a gain element;

FIG. 5 is a stability map for the laser swept source showing the SOAdrive current as function of wavelength for its scan band; and

FIG. 6 is a stability map for the laser swept source showing the SOAdrive current and tunable filter drive voltage as function of wavelengthshowing synchronous modulation of both elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 contains plots of the k-clock frequency and clock jitter of theswept optical signal from a passively swept mode locked laser during itsfrequency sweeping through the scan band where there is no activemodulation of the SOA current. The k-clock exhibits high levels ofjitter suggesting poor tuning performance. Further, the power output ofthe swept optical signal from tunable laser is highly unstable over thescan. RIN is also high. The spectrogram shows the existence of pulses inthe swept optical signal at approximately 2600 and 1300 MHz. The energydistribution seems to vary over the course of the scan through thetuning band of the laser.

This uncontrolled pulse behavior during the course of the sweep of theswept optical signal through the scan band is believed to be instigatedby the changing characteristics of the laser cavity over the scan band.

In uncontrolled lasers, the number of pulses in the laser cavity due toswept mode locking has been observed to change between forward andbackward sweeps, and can even switch in the middle of a sweep asillustrated.

FIGS. 2A and 2B are the results of a computer simulation. It shows atunable laser exhibiting passive swept mode locking without gainmodulation. In this case, the laser operates with 2 pulses per cavityround trip.

The correlation plots of FIG. 2B, one for light exiting the SOA gainelement and one for light exiting the tunable filter tuning element, arecomputer simulations of a swept source coherence length measurement, butcarried out to extreme path differences. The usual coherence lengthmeasurement occurs at path differences near zero (2). At 120 mm (3) thepulses are interfering with their neighbors. At 240 mm (4) the pulsesare interfering with pulses 1 cavity round trip away, which is twopulses apart.

These secondary coherences (3) (4) can sometimes be a problem inpractical OCT systems where small stray reflections at lengths nearlycorresponding to the cavity length or fractions thereof (depending onthe number of pulses per round trip) can produce artifacts in the OCTimage.

FIG. 3 shows an optical coherence analysis system 10, such as atomography system, which has been constructed according to theprinciples of the present invention.

An optical swept laser 100 generates the tunable or swept optical signalon optical fiber 110 that is transmitted to interferometer 550. Theswept optical signal scans over a scan band with a narrowband emission.

The swept laser 100 is generally intended for high speed tuning togenerate swept optical signals that repeatedly scan over the scanband(s) at rates of greater than 1 kiloHertz (kHz). In currentembodiments, the swept laser 100 tunes at speeds greater than 20 or 100kHz. In very high speed embodiments, the swept laser 100 tunes at speedsgreater than 200 or 500 kHz.

Typically, the width of the tuning or scan band is greater than 10nanometers (nm). In the current embodiments, it is preferably between 50and 150 nm, although even wider tuning bands are contemplated in someexamples. On the other hand, the bandwidth of the narrowband emissionhas a full width half maximum (FWHM) bandwidth of less than 20 or 10GigaHertz (GHz), and is usually 5 GHz or less. For optical coherencetomography, this high spectral resolution implies a long coherencelength and therefore enables imaging deeper into samples, for exampledeeper than 5 millimeters (mm). On the other hand, in lower performanceapplications, for example OCT imaging less than 1 mm deep into samples,broader FWHM passbands are sometimes appropriate, such as passbands ofabout 200 GHz or less.

The tuning speed can also be expressed in wavelength per unit time. Inone example, for an approximately 110 nm tuning band or scanband and 100kHz scan rate, assuming 60% duty cycle for substantially linearup-tuning, the peak sweep speed would be 110 nm*100 kHz/0.60=18,300nm/msec=18.3 nm/μsec or faster. In another example, for an approximately90 nm tuning range and 50 kHz scan rate, assuming a 50% duty cycle forsubstantially linear up-tuning, the peak sweep speed is 90 nm*50kHz/0.50=9,000 nm/msec=9.0 nm/μsec or faster. In a smaller tuning bandexample having an approximately 30 nm tuning range and 2 kHz scan rate,assuming a 80% duty cycle for substantially linear tuning, the peaksweep speed would be 30 nm*2 kHz/0.80=75 nm/msec=0.075 nm/μsec, orfaster.

Thus, in terms of scan rates, in the preferred embodiments describedherein, the sweep speeds are greater than 0.05 nm/μsec and preferablygreater than 5 nm/μsec. In still higher speed applications, the scanrates are higher than 10 nm/μsec.

A controller 510, via a digital to analog converter (DAC) 512, generatesa filter, or tunable element, drive waveform or waveform 516. Thistunable element drive signal 516 is amplified by amplifier 514 andapplied to the laser's tunable element. In one example, the controller510 stores the filter drive waveform that linearizes the frequency sweepfor one or more tunable optical filters, such as Fabry-Perot tunablefilters, tilting gratings, or other tunable optical elements, containedin the swept source system 100.

The controller 510, via the digital to analog converter (DAC) 512, alsogenerates a gain element drive waveform 520, which is amplified byamplifier 518 and applied to the laser's gain element. In one example,the controller 510 stores the gain element drive waveform thatstabilizes the frequency sweep by modulating the gain of the gainelement with the same periodicity and synchronously with the tunableelement drive signal 516. The gain element drive waveform 520 is appliedto the gain element, typically a semiconductor optical amplifier, of theswept laser 100. In other embodiments, it is applied to an intra cavityloss element that modulates the gain of the laser's cavity by applying atime varying loss.

A clock system 522 is used to generate k-clock signals at equally spacedoptical frequency sampling intervals as the swept optical signal istuned or swept over the scan or tuning band, in one embodiment.

In the illustrated example, a Mach-Zehnder-type interferometer 550 isused to analyze the optical signals from the sample 5. The swept opticalsignal from the optical swept laser 100 is transmitted on fiber 110 to a90/10 optical fiber coupler 552 or other beam splitter, to give specificexamples. The swept optical signal is divided between a reference arm556 and a sample arm 554 of the system 10.

The optical fiber of the reference arm 556 terminates at the fiberendface 564. The light 102R exiting from the reference arm fiber endface564 is collimated by a lens 568 and then reflected by a reference mirror566 to return back, in some exemplary implementations.

The reference mirror 566 has an adjustable fiber to mirror distance, inone example. This distance determines the depth range being imaged, i.e.the position in the sample 5 of the zero path length difference betweenthe reference arm 556 and the sample arm 554. The distance is adjustedfor different sampling probes and/or imaged samples. Light returningfrom the reference mirror 566 is returned to a reference arm circulator560 and directed to an interference signal combiner 570, such as a 50/50fiber coupler. In other examples, such as those using free space opticalconfigurations, the combiner 570 is a partially reflecting mirror/beamsplitter.

The fiber on the sample arm 554 terminates at the sample arm probe 562.The exiting swept optical signal 102S is focused by the probe 562 ontothe sample 5. Light returning from the sample 5 is returned to a samplearm circulator 558 and directed to the interference signal combiner 570.

The reference arm signal and the sample arm signal are combined or mixedin the interference signal combiner 570 to generate an interferencesignal.

The interference signal is detected by a detection system 580.Specifically, a balanced receiver, comprising two detectors 582, islocated at each of the outputs of the fiber coupler 570 in theillustrated embodiment. The electronic interference signal from thebalanced receiver 582 is amplified by amplifier 584, such as atransimpedance amplifier.

A data acquisition and processing system 586 of the detection system 580is used to sample the interference signal output from the amplifier 584.The k-clock signals derived from the clock system 522 are preferablyused by the data acquisition and processing system 586 to synchronizesystem data acquisition with the frequency tuning of the optical sweptlaser 100. In other examples, the swept laser tunes linearly infrequency and/or resampling is used.

In any case, the data acquisition and processing system 586 samples theinterference signals to generate evenly spaced samples of theinterference signal in the optical frequency domain. A sweep startsignal is preferably provided by the controller 510 when a new sweep isto begin.

A complete data set is collected of the sample 5 by spatially rasterscanning the focused probe beam point from the probe 562 over the sample5 in a Cartesian geometry x-y fashion or a cylindrical geometry theta-zfashion. The spectral response at each one of these points is generatedfrom the frequency tuning of the swept laser 100.

The data acquisition and processing system 586 performs a Fouriertransform on the data in order to reconstruct the image and perform a 2Dor 3D tomographic reconstruction of the sample 5. This transformed dataare displayed by the display system 590.

FIG. 4 shows passively mode-locked laser swept source 100 for opticalcoherence analysis, which has been constructed according to theprinciples of the present invention. This embodiment controls orstabilizes the mode-locked operation by modulating the bias current toan intracavity gain element synchronously and with the same periodicityof the sweeping of the swept optical signal through the scan band.

In the current embodiment, the laser swept source 100 is preferably alaser as generally described in incorporated U.S. Pat. No. 7,415,049 B1.It includes a linear cavity with a gain element 410 and a frequencytuning element 412, which are preferably implemented on a common opticalbench B. In the illustrated example, the frequency tuning element is areflective Fabry-Perot filter, which defines one end of the cavity, inthe illustrated implementation and thus also functions as an endreflector of the laser cavity.

In other embodiments, other cavity configurations are used such as ringcavities. Further other cavity frequency tuning elements are used suchas gratings and thin-film filters. In some examples, these tuningelements are mechanically tuned such as rotated or pivoted. Theseelements can also be located entirely within the cavity such as an angleisolated transmissive Fabry-Perot tunable filter or grating.

Currently, the passband of the Fabry-Perot filter 412 is between 1 and10 GHz and is tuned over a tuning band or scan band of greater than 10nanometers (nm), and is usually greater than 50 or 100 nm.

In more detail with respect to the current embodiment, the tunable laser100 comprises a semiconductor optical amplifier gain chip 410 that ispaired with a micro-electro-mechanical (MEMS) angled reflectiveFabry-Perot tunable filter 412, which defines one end of the lasercavity. The cavity extends to a second output reflector 405 that islocated at the end of a fiber pigtail 406 that is coupled to the bench Band also forms part of the cavity.

Currently, the length of the cavity is at least 40 millimeters (mm) longand preferably over 50 to 80 mm. This ensures close longitudinal modespacing that reduces mode hopping noise.

In other embodiments, shorter cavities are used. In some of theseembodiments, very short cavities with wider passband tuning elements(filters) 412 are used for extremely high speed applications where onlyshort coherence lengths are required. In some of these examples, thepassband of the Fabry-Perot filter 412 is between 20 and 40 GHz, orwider. The length of the laser cavity is less than 20 mm or 10 mm, andthus may not extend into optical fiber, but is entirely implemented onthe bench B.

The tunable or swept optical signal passing through the output reflector405 is transmitted on optical fiber 110 or via free space to theinterferometer 550 of the OCT system.

The semiconductor optical amplifier (SOA) chip gain element 410 islocated within the laser cavity. In the current embodiment, input andoutput facets of the SOA chip 410 are angled and anti-reflection (AR)coated, providing typically parallel beams from the two facets. In thepreferred embodiment, the SOA chip 410 is bonded or attached to thecommon bench B via a submount.

The material system of the chip 410 is selected based on the desiredspectral operating range. Common material systems are based on III-Vsemiconductor materials, including binary materials, such as GaN, GaAs,InP, GaSb, InAs, as well as ternary, quaternary, and pentenary alloys,such as InGaN, InAlGaN, InGaP, AlGaAs, InGaAs, GaInNAs, GaInNAsSb,AlInGaAs, InGaAsP, AlGaAsSb, AlGaInAsSb, AlAsSb, InGaSb, InAsSb, andInGaAsSb. Collectively, these material systems support operatingwavelengths from about 400 nanometers (nm) to 2000 nm, including longerwavelength ranges extending into multiple micrometer wavelengths.Semiconductor quantum well and quantum dot gain regions are typicallyused to obtain especially wide gain and spectral emission bandwidths.Currently, edge-emitting chips are used although vertical cavity surfaceemitting laser (VCSEL) chips are used in different implementations.

The use of a semiconductor chip gain medium 410 has advantages in termsof system integration since semiconductor chips can be bonded tosubmounts that in turn are directly bonded to the bench B.

Each facet of the SOA 410 has an associated lens structure 414, 416 thatis used to couple the light exiting from either facet of the SOA 410 inthe illustrated free-space version of the laser. The first lensstructure 414 couples the light between the back facet of the SOA 410and the reflective Fabry-Perot tunable filter 412. Light exiting out theoutput or front facet of the SOA 410 is coupled by the second lensstructure 416 to a fiber end facet of the pigtail 406.

In a current implementation, each lens structure comprises a LIGAmounting structure, which is deformable to enable post installationalignment, and a transmissive substrate on which the lens is formed. Thetransmissive substrate is typically solder or thermocompression bondedto the mounting structure, which in turn is solder bonded to the opticalbench B.

The fiber facet of the pigtail 406 is also preferably mounted to thebench B via a fiber mounting structure, to which the fiber 406 is solderbonded. The fiber mounting structure is likewise usually solder bondedto the bench B. Deformable fiber mounting structure typically allowspost installation alignment of the fiber for optimal coupling of lightfrom the SOA chip to the fiber.

The angled reflective Fabry-Perot filter 412 is a multi-spatial-modetunable filter that provides angular dependent reflective spectralresponse back into the laser cavity. This characteristic is discussed inmore detail in incorporated U.S. Pat. No. 7,415,049 B1.

Preferably, the tunable filter 412 is a Fabry-Perot tunable filter thatis fabricated using micro-electro-mechanical systems (MEMS) technologyand is attached, such as directly solder bonded, to the bench B.Currently, the filter 412 is manufactured using technologies describedin U.S. Pat. No. 6,608,711 or 6,373,632, which are incorporated hereinby this reference. A curved-flat resonator structure is used in which agenerally flat mirror and an opposed curved mirror define a filteroptical cavity, the optical length of which is modulated byelectrostatic deflection of at least one of the mirrors.

The passively mode-locked laser swept source 100 and the otherembodiments discussed hereinbelow are generally intended for high speedtuning to generate tunable swept optical signals that scan over thetuning band or scanband at speeds greater than 1 kiloHertz (kHz). Incurrent embodiments, the laser swept source 100 tunes over the scanbandat speeds greater than 50 or 100 kHz. In very high speed embodiments,the mode-locked laser swept source 100 tunes at speeds greater than 200or 500 kHz.

The controller 510 provides a tuning voltage function to the Fabry-Perotfilter 412, which includes a membrane that is electrostaticallydeflectable to thereby sweep the filter optical passband across thetuning band, preferably with optical frequency varying linearly withtime. Typically, the width of the scan or tuning band is greater than 10nm. In the current embodiments, it is preferably between 50 and 150 nm,although even wider tuning bands are contemplated in some examples.

In one implementation, an extender element 415 is added to the lasercavity. This is fabricated from a transparent, preferably highrefractive index material, such as fused silica, silicon, GaP or othertransmissive material having a refractive index of ideally about 1.5 orhigher. Currently silicon or GaP is preferred. Both endfaces of theextender element 415 are antireflection coated. Further, the element 415is preferably angled by between 1 and 10 degrees relative to the opticalaxis of the cavity to further spoil any reflections from the endfacesfrom entering into the laser beam optical axis.

The extender element 415 is used to change the optical distance betweenthe laser intracavity spurious reflectors and thus change the depthposition of the spurious peaks in the image while not necessitating achange in the physical distance between the elements.

The bench B is termed a micro-optical bench and is preferably less than10 millimeters (mm) in width and about 25 mm in length or less. Thissize enables the bench to be installed in a standard, or nearstandard-sized, butterfly or DIP (dual inline pin) hermetic package. Inone implementation, the bench B is fabricated from aluminum nitride. Athermoelectric cooler is disposed between the bench B and the package(attached/solder bonded both to the backside of the bench and innerbottom panel of the package) to control the temperature of the bench B.The bench temperature is detected via a thermistor installed on thebench B.

The system and method for stabilizing the mode locked swept laser system100 of the illustrated embodiment utilizes a controlled drive current tothe cavity gain element, SOA 410.

The stable operating regime for passively mode locked swept lasers is abalance between sweep rate, filter bandwidth, cavity losses and gaincharacteristics in the SOA 410. Unfortunately, these parameters criticalto the operation of the laser exhibit significant variation withwavelength, and the wavelength dependence of each of these parametersarises from separate, unrelated characteristics of the system and itscomponents. As such, the variations in the parameters do not, ingeneral, exhibit any simple relationship nor do they necessarily vary ina way that consistently supports stable operation over a wide tuningrange. This creates significant challenges in the design and manufactureof high speed swept tunable lasers which cover a range of wavelengths.

To ensure stable operation over a wide range of conditions an optimallaser design would match, for example, a wider filter linewidth with afaster instantaneous sweep rate and a narrower filter linewidth with aslower sweep rate. However, the linewidth for the Fabry-Perot filter 412typically grows wider towards the edges of the tuning range where theinstantaneous tuning rate by necessity is substantially reduced. This isin direct contrast to the optimal design, and results in very tighttolerances for the linewidth variations in the tunable filter 412.

The variation in the gain characteristics of the SOA 410 with wavelengthcan be even more complex, and places additional constraints on thestable range of filter linewidths and sweep rates. This is influenced,for example, by the strong drop in gain near the band edges of the SOA412, or, in the case of a quantum well SOA, by the number of energylevel transitions involved in establishing the gain spectrum. Thesevariations, both in the gain itself and in the dependence of the gain ondrive current, cause shifts in the optimal filter linewidth for stableperformance across the wavelength scan band.

To overcome these limitations, the bias point (injected drive current)of the SOA 410 is modulated during the laser sweep in a manner thatcompensates for the variation in the gain characteristics in the SOA 410as well as variations in the sweep rate and filter linewidth across thewavelength band of interest.

FIG. 5 shows a stability map, such as in the figure below, illustratingthe stable (black areas) and unstable (white areas) operating range as afunction of SOA drive current and wavelength. In general, the optimaldrive current for each wavelength in the scan band is not known apriori, but is evaluated empirically.

In the illustrated map, there is no constant bias current on the SOA 410that results in stable operation over the entire wavelength range shown.

FIG. 6 shows the stability map plotted with the drive voltage to thetunable filter 410 over the scan band stretching from less than 1000 nmto over 1100 nm. By modulating the drive current 520 synchronously andwith the periodicity of the drive signal 516 to the tunable filter 412along the path illustrated (dotted line), stable operation of the laser100 over the scan band is achieve with good operating margin. Inpractice this is done by synchronizing the SOA current modulation drivewaveform 520 with the filter sweep drive waveform 516. Both of thesewaveforms being stored as lookup tables, for example, by the controller510.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. For example, although the inventionhas been described in connection with an OCT or spectroscopic analysisonly, the invention could also be applied along with IVUS, FLIVUS, HIFU,pressure sensing wires and image guided therapeutic devices.

What is claimed is:
 1. An optical coherence imaging method, comprising:providing a laser swept source to generate a swept optical signal;modulating a drive signal for a gain element as the swept optical signalis swept through a scan band by modulating a current to the gain elementsynchronously with and with the periodicity of a tunable element drivesignal that is applied to a tunable element within a cavity of the laserswept source, which tunable element is used to control a wavelength ofthe swept optical signal, to control a mode-locked operation of thelaser swept source; transmitting the swept optical signal to aninterferometer having a reference arm and a sample arm, in which asample is located; combining the swept optical signal returning from thesample arm and the reference arm to generate an interference signal;detecting the interference signal; and generating image information ofthe sample from the detected interference signal.
 2. An opticalcoherence analysis system comprising: a swept laser source forgenerating a swept optical signal that is frequency tuned over a scanband, including a gain element for amplifying light within a lasercavity of the swept laser source to generate the swept optical signaland a tuning element for controlling a frequency of the swept opticalsignal to sweep across the scan band; a controller for modulating acurrent signal for the gain element as the swept optical signal is sweptthrough the scan band synchronously with and with the periodicity of atuning element drive signal to the tuning element within the lasercavity of the laser swept source, to control a mode-locked operation ofthe laser swept source; an interferometer for dividing the swept opticalsignal between a reference arm and a sample arm leading to a sample; anda detector system for detecting an interference signal generated fromthe swept optical signal from the reference arm and from the sample arm.3. A method as claimed in claim 1, further comprising: generating astability map as a function of wavelength for the laser swept source;and modulating the current to the gain element based on the stabilitymap.
 4. A method as claimed in claim 1, wherein the laser swept sourcetunes at greater than 5 nanometers per microsecond.
 5. A method asclaimed in claim 1, wherein the scan band is greater than 50 nanometers.6. A method as claimed in claim 1, further comprising a controllerstoring a gain element drive waveform that defines the current to thegain element for the sweep through the scan band.
 7. A method as claimedin claim 6, further comprising synchronizing the gain element drivewaveform with a filter drive waveform that defines the tunable elementdrive signal for the sweep through the scan band.
 8. A method as claimedin claim 1, wherein the tunable optical element is a Fabry-Perot tunablefilter.
 9. An optical coherence analysis system comprising: a sweptlaser source for generating a swept optical signal that is frequencytuned over a tuning band, including a gain medium for amplifying lightwithin a laser cavity of the swept laser source to generate the sweptoptical signal and a tuning element for controlling a frequency of theswept optical signal to sweep across a scanband; a controller formodulating a drive signal for the gain element as the swept opticalsignal is swept through the scan band synchronously with a tuningelement drive signal to the tuning element; an interferometer fordividing the swept optical signal between a reference arm and a samplearm leading to a sample; and a detector system for detecting aninterference signal generated from the swept optical signal from thereference arm and from the sample arm.
 10. A system as claimed in claim9, wherein the controller controls the mode-locked operation of theswept laser source by modulating bias current to the gain medium, whichis a semiconductor optical amplifier.
 11. A system as claimed in claim9, wherein the laser swept source tunes at greater than 5 nanometers permicrosecond.
 12. A system as claimed in claim 9, wherein the scan bandis greater than 50 nanometers.
 13. A system as claimed in claim 9,wherein the controller stores a gain element drive waveform that definesthe current signal for the gain element for the sweep through the scanband.
 14. A system as claimed in claim 13, wherein the controllersynchronizes the gain element drive waveform with a filter drivewaveform that defines the tuning element drive signal for the sweepthrough the scan band.
 15. A system as claimed in claim 9, wherein thetuning element is a Fabry-Perot tunable filter.